Sorry to hear. Monopterus are usually very hardy. But yes, as they get older, they do get stressed easily & when they got stressed, they are more prone to diseases, hunger strikes and "freedom syndrome" (wanting to escape the aquarium all the times).
I bought mine almost 3 years ago. When it was newly bought, mine was very active and ate like a pig; live food or frozen food are fair game for it. But after a year, and it became a bit bigger and longer, it started to show signs of stress, it doesn't eat as much as before, it doesn't swim around as actively as before, and I found it climbing the filter very often, as if wanting to escape the aquarium (luckily I have taken precautions). From an active, aggressive predator, it has become a fat old fish whose daily activity consists of hanging upside down from my filter and stick its head to the sand. I am planning to rehouse it in a pond sometime later, or to donate it to a friend who has a larger aquarium.